Ramsay became a midshipman in the Royal Navy in 1899 and commanded a destroyer in World War I. During the interwar years he alternated periods of active duty with assignments on the staff of the Naval War College (1927–29) and the Imperial Defence College (1931–33). He became a rear admiral in 1935 and retired as a vice admiral in 1938. When World War II began, he was made flag officer at the Channel port of Dover. With the collapse of the Allied front in northern France in June 1940, Ramsay was put in charge of organizing the evacuation of the British Expeditionary Force from Dunkirk. Mobilizing every usable military and civilian craft he could find, he oversaw the evacuation of about 338,000 British and other Allied soldiers from Dunkirk and nearby beaches, bringing them to safety in England. He was knighted for this achievement.

Ramsay commanded several major amphibious assault operations in the next two years. Working under Admiral Andrew Cunningham, Ramsay helped to oversee the fleet that covered the American landings in North Africa (Operation Torch; November 1942) and then commanded the British landing operations in the Allied amphibious assault on Sicily (July 1943). In 1943 Ramsay was appointed naval commander in chief for Operation Overlord, the projected Allied invasion of northern France. The ships under his command landed one million Allied troops in France in one month starting from D-Day (June 6, 1944).

Ramsay had been made an admiral shortly prior to the invasion. He was killed in an airplane crash while on his way to meet British Field Marshal Bernard Montgomery in Brussels. His 1944 diary was posthumously edited by Robert Love, Jr., and John Major and published as The Year of D-Day (1994).